<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        div {
            width: 100px;
            height: 100px;
            background-color: black;
            float: left;
            margin-right: 10px;
        }
    </style>
</head>

<body>
    <button onclick="red()">变色</button>
    <div class="div"></div>
    <div class="div"></div>
    <div class="div"></div>

    <script>
        var div = document.getElementsByClassName('div')

        function red() {
            // alert(div.length)
            for (var i = 0; i <= div.length; i++) {
                div[i].style.backgroundColor = 'red'
            }
        }
    </script>
</body>

</html>